Hybrid photonics-solid state quantum computer

ABSTRACT

There is described herein a quantum computing system, quantum processor, and method of operating a quantum computing system. The quantum computing system comprises a quantum control system configured for at least one of delivery and receipt of multiplexed optical signals. At least one optical fiber is coupled to the quantum control system for carrying the multiplexed optical signals, and a quantum processor is disposed inside a cryogenics apparatus and coupled to the at least one optical fiber. The quantum processor comprises: at least one converter configured for converting between the multiplexed optical signals and microwave signals at different frequencies; and a plurality of solid-state quantum circuit elements coupled to the at least one converter and addressable by respective ones of the microwave signals at different frequencies.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 17/499,925 filed on Oct. 13, 2021, which claims the benefit of U.S.Provisional Patent Application No. 63/124,761 filed on Dec. 12, 2020 andU.S. Provisional Patent Application No. 63/225,963 filed on Jul. 27,2021, the contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present disclosure generally relates to the field of quantumcomputing. More specifically, the present disclosure relates to controland readout of solid-state qubits such as superconducting qubits, spinqubits, and topological qubits.

BACKGROUND OF THE ART

Solid-state qubits such as superconducting circuits, spin qubits andtopological qubits based on semiconductor/superconductor nanowires areamong leading architectures to build a quantum computer. Quantum controland readout of such qubits typically involve use of electronics andwaves in microwave frequency (GHz) regimes.

To protect the qubits against thermal noise, these qubits are placed incryogenics systems and operated in ultra-low temperatures, typically ofthe order of few millikelvins above absolute zero.

The qubits are controlled and measured by generating microwave pulses atroom temperature and delivering the waves to the qubits in the cryostatvia a set of microwave coaxial cables. When measuring the qubits, theinput microwave pulses interact with the qubit circuit to produce anoutput microwave signal which is then transmitted to readout electronicsby another coaxial cable. The coaxial cables are thermally anchored todifferent cooling stages of the cryogenics system. A quantum processorusually requires at least one control coaxial line per qubit forperforming single-qubit gates on top of a number of readout coaxiallines. Additional coaxial lines can also be required to control couplersused to implement multi-qubit gates.

While existing techniques for controlling and measuring qubits aresuitable for their purposes, improvements are desired.

SUMMARY

In accordance with a first broad aspect, there is provided a quantumcomputing system comprising a quantum control system configured for atleast one of delivery and receipt of multiplexed optical signals. Atleast one optical fiber is coupled to the quantum control system forcarrying the multiplexed optical signals, and a quantum processor isdisposed inside a cryogenics apparatus and coupled to the at least oneoptical fiber. The quantum processor comprises: at least one converterconfigured for converting between the multiplexed optical signals andmicrowave signals at different frequencies; and a plurality ofsolid-state quantum circuit elements coupled to the at least oneconverter and addressable by respective ones of the microwave signals atdifferent frequencies.

In accordance with another broad aspect, there is provided a quantumprocessor comprising at least one substrate, the at least one substratehaving fabricated thereon solid-state quantum circuit elements,microwave circuit elements, and integrated photonic elements on a sameone or different ones of the at least one substrate, the integratedphotonic elements connectable to at least one optical fiber for datatransmission.

In accordance with yet another broad aspect, there is provided a methodfor operating a quantum computing system. Uplink microwave signals atdifferent frequencies are converted into uplink multiplexed opticalsignals. The uplink multiplexed optical signals are delivered, via atleast one optical fiber, to a quantum processor comprising solid-statequantum circuit elements. The uplink multiplexed optical signals arereconverted, at the quantum processor, to the uplink microwave signalsat different frequencies and the solid-state quantum circuit elementsare addressed with the uplink microwave signals as reconverted, whereinthe different frequencies are used to address different ones of thesolid-state quantum circuit elements.

Features of the systems, devices, and methods described herein may beused in various combinations, in accordance with the embodimentsdescribed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a solid-state quantum computing system in accordance withthe prior art;

FIG. 2 shows an example embodiment of a system which uses aphotonics-based system to control or measure an array of solid-statequbits;

FIG. 3 shows an example embodiment of a system which uses multiplexingto control or measure an array of solid-state qubits using a singleoptical fiber link;

FIG. 4 shows an example embodiment of a system which uses modulation ofoptical frequency combs as a multiplexing method to control or measurean array of solid-state qubits using a single optical fiber link;

FIG. 5 shows an example embodiment of a system which uses an opticalarbitrary wave generator to achieve multiplexing to control or measurean array of solid-state qubits using a single optical fiber link;

FIG. 6 shows an example of a system which uses an amplifier to amplifythe input control or readout signal after it has been converted fromoptical frequencies to microwave frequencies.

FIG. 7 shows an example of a system which uses a single coaxial cable tosupply a microwave pump signal to an array of amplifiers.

FIG. 8 shows an example of a quantum processor in which opticalcircuitry and qubits are integrated on the same substrate; and

FIG. 9 shows an example of a multi-chip quantum processor in whichqubits are fabricated on one substrate and optical circuitry isfabricated on a second substrate.

DETAILED DESCRIPTION

The present disclosure is directed to a scalable solid-state quantumcomputing platform where the quantum processor is operated at ultra-lowtemperatures and the need to use a significant number of coaxial cablesis alleviated. The connection between the quantum processor and thequantum control system is achieved using optical fibers, and sendingcontrol or readout pulses to the solid state quantum processor operatedat ultra-low temperatures uses a photonics approach.

FIG. 1 shows an example embodiment of a solid-state quantum computingsystem 100 in accordance with the prior art. The system 100 comprises aquantum processor 110 placed inside a cryogenics apparatus 120 such as adilution fridge, and cooled down to ultra-low temperatures. Control andreadout of the qubits in the quantum processor 110 is performed by aquantum control system 160. The quantum control system 160 may itselfhave separate modules for uplink and downlink. As used herein, anysignal originating from the quantum control system 160 and delivered tothe quantum processor 110, either for qubit control or readout, isreferred to as “uplink” and any signal originating from the quantumprocessor 110 and delivered to the quantum control system 160 isreferred to as “downlink”. In FIG. 1, an uplink module 140 generates thenecessary microwave pulses for qubit control and readout. The microwavepulses are transmitted to the quantum processor 110 via a set of coaxiallines 170A that are thermally anchored to cold stages 121 of thecryogenic apparatus 120 through a set of attenuators 122. Qubit statesare measured through a set of coaxial lines 170B that leave the quantumprocessor 110 and connect to the downlink module 150 which is at roomtemperature and outside the cryogenics apparatus 120. The set of coaxiallines 170B are also thermally anchored to the cold stages 121 of thecryogenics apparatus 120 through the set of attenuators 122. This qubitreadout chain may also involve one or more amplifiers 180 and/orcirculators 190 to further improve the signal to noise ratio and protectthe qubits from microwave feedback.

The quantum processor 110 requires at least one control coaxial line perqubit on top of a number of readout coaxial lines. Additional coaxiallines can also be required to control couplers used to implementmulti-qubit gates. These coaxial lines are bulky, expensive, and alsoresult in heat leak from the hotter stages of the cryogenics apparatus120 to the colder areas. Noting that the cooling power of the cryogenicsapparatus 120 decreases with temperature, installing more than a fewhundred coaxial lines is challenging as the heat leak from the coaxiallines may exceed the cooling power of the cryogenics apparatus 120.

Moreover, practical applications of quantum computers typically requirehundreds of thousands, and even millions, of qubits. Therefore, thesolid-state quantum computing system 100 in accordance with the priorart is not scalable for large scale quantum processors. The presentdisclosure addresses these shortcomings by making use of optical fibers,which result in significantly lower heat load compared to coaxialcables. Optical fibers also provide large bandwidth which allows the useof multiplexing to address a large number of qubits with a single fiber.

FIG. 2 illustrates an example quantum computing system 200 in accordancewith the present disclosure. The system 200 comprises a quantumprocessor 210 which is housed inside a cryogenics apparatus 220 such asa dilution fridge. The quantum processor 210 comprises one or moreoptical-to-microwave converters 211 which down-convert optical signalsto microwave signals. The microwave signals are delivered to one or morequantum circuit elements 212 such as solid-state qubits and couplersoperating in a sub-Tera Hertz frequency band. Microwave signalsoriginating from quantum circuit elements 212 are up-converted tooptical frequencies by one or more microwave-to-optical converters 213.The quantum processor 210 is connected to a quantum control system 260through one or more fiber optic links 230. The quantum control system260 may itself have separate subsystems for uplink, such as uplinkmodule 240, and downlink, such as downlink module 250. In someembodiments, the quantum control system 260 comprises one integratedsystem for uplink and downlink. In some other embodiments, the quantumcontrol system 260 comprises physically separate subsystems for uplinkand downlink.

Optical-to-microwave converters 211 may for example consist ofphotodiodes, such as InGaAs photodiodes, or single-photon detectors.Microwave-to-optical converters 213 may for example consist of opticalphase modulators, such as LiNbO₃-based electro-optical phase modulators,or other transducers based on optomechanics, piezo-optomechanics,electrooptics or magneto-optics.

In some embodiments, parts or all of the quantum control system 260and/or its subsystems may reside inside the cryogenics apparatus 220.

Instead of pulse shaping microwave signals and delivering them to aquantum processor through coaxial cables, the quantum control system 260modulates and demodulates optical signals that are delivered to ororiginate from the quantum processor 210 through optical fibers 230. Theoptical signals may have a wavelength in the short-wave infrared band,for which the transmission of optical fibers is maximal.

In some embodiments, one or more multiplexing schemes, such asWavelength-Division Multiplexing (WDM), may be used to control and/ormeasure multiple qubits at the same time. FIG. 3 illustrates an exampleembodiment of the uplink side of a quantum computing system 300implementing a multiplexing scheme. In this example, an uplink module340 comprises an array of optical sources 341 followed by an array ofmodulators 342 before the optical signals are combined using amultiplexer into an optical fiber 330 and multiplexed optical signalsare delivered to a quantum processor 310 located inside a cryogenicsapparatus 320.

FIG. 4 illustrates another example of the uplink side of a quantumcomputing system 400 where an uplink module 440 uses a frequency combsource 441 rather than an array of optical sources. In one exampleimplementation, the frequency comb source 441 may comprise a mode-lockedlaser which is self-referenced. In another example implementation, thefrequency comb source 441 may rely on strong electro-optic phasemodulation of a continuous laser to generate the frequency comb. In yetanother example implementation, the frequency comb source 441 maycomprise a light source such as a continuous laser connected to anonlinear (Kerr) micro-resonator which creates a frequency comb throughnonlinear mixing. In all of these examples, multiplexed optical signalsare used to address multiple qubits, whereby signals of differentfrequencies are used to address different qubits.

In one implementation, the frequency comb generated by the source 441may pass through a demultiplexer 442 which separates the comb lines androutes them to individual modulators 443 (e.g. a Mach-Zehnder modulator)for pulse shaping. Each frequency line in the frequency comb may beintended for preforming an operation on an individual quantum circuitelement (e.g. a qubit or a coupler) located in a quantum processor 410.The individual modulators 443 are used to provide pulse shaping onindividual frequencies according to the particular operation intended tobe performed on a respective quantum circuit element in the quantumprocessor 410. The channels are then recombined by a multiplexer 444before transmission over an optical fiber 430 to the quantum processor410 located inside a cryogenics apparatus 420.

FIG. 5 illustrates another example embodiment of the uplink side of aquantum computing system 500 in which an uplink module 540 comprises afrequency comb source 541 and an optical arbitrary wave generator (OAWG)542, such as a line-by-line pulse shaper, which translates quantumoperations to waveforms. The multiplexed optical signal output by thegenerator 542 is delivered via an optical fiber 530 to a quantumprocessor 510 located inside a cryogenics apparatus 520.

FIG. 6 illustrates another example embodiment of the uplink side of aquantum computing system 600 which allows the use of a lower opticalpower in an uplink module 640 and optical fibers 630 to avoid excessiveheating in a quantum processor 610 located inside a cryogenics apparatus620. Since the passive heat load (i.e. due to heat propagating along thefiber) of optical fibers is negligible, what may limit the scalabilityof optically controlled quantum computers is the active heat load, i.e.heat due to the dissipation of the optical power at the exit of theoptical fiber. The quantum processor 610 comprises a microwave amplifier614 between an optical-to-microwave converter 611 and quantum circuitelements 612. In some embodiments, the microwave amplifier 614 can be aquantum-limited parametric amplifier, such as a Josephson ParametricAmplifier (JPA) or a Travelling-Wave Parametric Amplifier (TWPA), whichare designed not to introduce any additional noise.

FIG. 7 illustrates a system 700 having a plurality of optical fibers 730and a plurality of optical-to-microwave converters 711A, 711B, 711C. Themicrowave signal generated by each optical-to-microwave converter 711A,711B, 711C is amplified by a respective amplifier 714A, 714B, 714Cbefore being directed to quantum circuit elements 712 of a quantumprocessor 710 located inside a cryogenic apparatus 720. To minimize thenumber of coaxial cables in the cryogenic apparatus 720, a singlecoaxial cable 750 can be used to supply a microwave pump signal to theamplifiers 714A, 714B, 714C. In FIG. 7, an uplink module 740 providesboth control optical signals and the microwave pump signal, but thesesignals could also be provided by different and separate modules.Alternatively, with an all-optical quantum control system, the microwavepump signal could also be generated from an optical signal provided byoptical fibers and down converted to microwave frequencies by anoptical-to-microwave converter.

FIG. 8 depicts a quantum processor 800 according to one embodiment. Thequantum processor 800 comprises at least one substrate 810. Solid-statequantum circuit elements, such as qubits and couplers, are fabricated onthe substrate through a series of nanofabrication techniques such aslithography, deposition, etching, and lift off. In one exampleembodiment, these quantum circuit elements are fabricated on one side ofthe substrate as part of layer 820. The quantum processor 800 alsocomprises microwave circuit elements, such as readout resonators,microwave filters and transmission lines, which may also be fabricatedon the substrate 810 through a series of nanofabrication techniques suchas lithography, deposition, etching, and lift off. The microwave circuitelements may be fabricated on either side of the substrate 810 and/or aspart of layer 820. The quantum processor 800 may also include anotherset of integrated photonic element such as waveguides, ring resonators,and optical-to-microwave converters. These photonics elements may befabricated in layer 830, on the same side of the substrate as thequantum circuit elements, and/or on the opposite side of the substratein layer 840. In the case where there are elements on both sides of thesubstrate, the electrical connection between both sides of the substratemay be achieved using vias 850. The integrated photonics circuitry maybe connected to a single or an array of optical fibers 860A, 860B fordata transmission.

FIG. 9 depicts a quantum processor 900 according to another embodiment.The quantum processor comprises at least one multi-chip module (MCM)970. In this arrangement, quantum circuit elements may be fabricated inlayer 940 on a first substrate 910 while another substrate 920 carriesanother segment of circuitry and elements such as photonic elements inlayer 960. Microwave circuit elements may reside on either substrate910, 920, in layer 940 and/or layer 950. Elements fabricated ondifferent substrates in layer 940 and layer 950 are electricallyconnected through a plurality of bond bumps 930, such as superconductingbond bumps. The multi-chip module 970 can also include additionalsubstrates, for example a third substrate hosting one or a plurality ofamplifiers (not shown). Amplifiers may also be fabricated on substrate920, on either side thereof. It will be understood that other variantsof 3D integration of such multi-chip quantum processors are possible,including but not limited to multi-chip vertical stack, flip-chip, anddie on wafer arrangements.

The above description is meant to be exemplary only, and one skilled inthe art will recognize that changes may be made to the embodimentsdescribed without departing from the scope of the disclosure. Stillother modifications which fall within the scope of the presentdisclosure will be apparent to those skilled in the art, in light of areview of this disclosure.

Various aspects of described herein may be used alone, in combination,or in a variety of arrangements not specifically discussed in theembodiments described in the foregoing and is therefore not limited inits application to the details and arrangement of components set forthin the foregoing description or illustrated in the drawings. Forexample, aspects described in one embodiment may be combined in anymanner with aspects described in other embodiments. The scope of thefollowing claims should not be limited by the embodiments set forth inthe examples, but should be given the broadest reasonable interpretationconsistent with the description as a whole.

1.-13. (canceled)
 14. A quantum processor comprising at least onesubstrate, the at least one substrate having fabricated thereonsolid-state quantum circuit elements, microwave circuit elements, andintegrated photonic elements on a same one or different ones of the atleast one substrate, the integrated photonic elements connectable to atleast one optical fiber for data transmission.
 15. The quantum processorof claim 14, wherein the solid-state quantum circuit elements, themicrowave circuit elements, and the integrated photonic elements arefabricated on the same one of the at least one substrate.
 16. Thequantum processor of claim 14, wherein the at least one substratecomprises a plurality of substrates arranged in a multi-chip module. 17.The quantum processor of claim 14, wherein the microwave circuitelements comprise microwave amplifiers.
 18. The quantum processor ofclaim 14, wherein the solid-state quantum circuit elements aresuperconducting qubits.
 19. (canceled)
 20. (canceled)
 21. The quantumprocessor of claim 14, wherein the at least one substrate has a firstside and a second side opposite to the first side, and the solid-statequantum circuit elements are on the first side.
 22. The quantumprocessor of claim 21, wherein the microwave circuit elements are on thesecond side of the at least one substrate.
 23. The quantum processor ofclaim 21, wherein the microwave circuit elements are on the first sideof the at least one substrate.
 24. The quantum processor of claim 21,wherein the integrated photonic elements are on the first side of the atleast one substrate.
 25. The quantum processor of claim 24, wherein theintegrated photonic elements are on a first layer on the first side ofthe at least one substrate, and the solid-state quantum circuit elementsare on a second layer on top of the first layer.
 26. The quantumprocessor of claim 21, wherein the integrated photonic elements are onthe second side of the at least one substrate.
 27. The quantum processorof claim 16, wherein the plurality of substrates comprises a firstsubstrate having a first side and a second side and a second substratehaving a third side and a fourth side, the first side of the firstsubstrate facing the third side of the second substrate.
 28. The quantumprocessor of claim 27, wherein the quantum state circuit elements are ona first layer on the first side of the first substrate.
 29. The quantumprocessor of claim 28, wherein the integrated photonic elements are on asecond layer on the fourth side of the second substrate.
 30. The quantumprocessor of claim 28, wherein the integrated photonic elements are on asecond layer on the third side of the second substrate.
 31. The quantumprocessor of claim 28, wherein the microwave circuit elements are on asecond layer on the third side of the second substrate.
 32. The quantumprocessor of claim 28, wherein the microwave circuit elements are on thefirst layer on the first side of the first substrate.
 33. The quantumprocessor of claim 27, wherein at least a first subset of the quantumcircuit elements, the microwave circuit elements, and the integratedphotonic elements are on the first side of the first substrate and atleast a second subset of the quantum circuit elements, the microwavecircuit elements, and the integrated photonic elements are on the thirdside of the second substrate.
 34. The quantum processor of claim 33,wherein the first subset and the second subset are electricallyconnected through superconducting bond bumps.
 35. The quantum processorof claim 27, wherein the quantum circuit elements, the microwave circuitelements, and the integrated photonic elements are on the first side ofthe first substrate, the third side of the second substrate, and thefourth side of the second substrate, wherein the first side and thethird side are electrically connected through bond bumps, and whereinthe third side and the fourth side are electrically connected throughvias through the second substrate.